The differentiation of L. major promastigotes from a non-infective to an infective or metacyclic stage is accomplished by changes in surface carbohydrates which can be detected by the lectin peanut agglutinin (PNA). On non-infective, logarithmic phase promastigotes, PNA binds to the surface lipophosphoglycan (LPG). During metacyclogenesis, the LPG is developmentally modified such that it no longer binds PNA, it expresses a novel carbohydrate epitope, and it is substantially increased in size. A major difference between the two forms can be localized to the composition of the tetrasaccharide subunits of the phosphorylated carbohydrates. The manner in which this structural change promotes complement resistance and intracellular survival is being investigated. The major acceptor molecular for C3b deposition on metacyclics is the LPG, which remain resistant to lysis because the membrane attack complex, while formed, is released from the surface. Efficient deposition of C3b in the absence of lysis promotes uptake of opsonized promastigotes by macrophage CR1 receptors.